Circuit for the continuous watching of the grounding resistance of electrical appliances

ABSTRACT

A circuit for the continuous watching of the grounding resistor of electrical appliances whose electrically conductive parts which may be touched, such as especially the casing are connected to a ground potential by a protecting conductor, one of the live leads (R,N) of the appliance (12) being connected with the protecting conductor by way of a series connection of at least one precision resistor (R1) for measuring purposes, one test pulse generator (TG), and one capacitor (C B ).

The invention relates to a circuit for continuously watching thegrounding resistor of electrical appliances whose touchable,electrically conductive structural members, such as especially thecasing, are connected to a ground potential by a protecting conductor.

As is well known, the metal parts to be touched, like the housing ofelectrical household appliances are connected to ground so as to protectthe user from electric shock. If an electrically conductive part of suchan appliance should become hot because of faulty insulation, the userwho touches that part still does not get an electric shock because thepart is at ground potential.

With the state of the art two fundamentally different types ofprotective circuits are to be distinguished, namely the so-called"protecting conductor connection" and the so-called "neutralization".

With the protecting conductor connection there is no galvanic connectionat or in the electrical appliance between the so-called neutralconductor of the power current network and the local reference ground.Rather, the appliance is furnished with a protecting conductor whichconnects all dangerous touchable metallic parts with the local referenceground. If a fault occurs in the insulation of the appliance, a faultcurrent which reaches the casing of the appliance is shunted to thereference ground by way of the protecting conductor. If this shuntedcurrent reaches a certain threshold value, usually the current supply tothe appliance is turned off by means of a fuse.

On the other hand, if the so-called neutralization is provided toprotect the user, the neutral conductor of the supply network at theelectrical appliance is connected to the local reference ground. Theprotecting conductor in this case need not be connected separately withthe reference ground, instead it is connected to the neutral conductorof the supply network at the socket (or in the terminal box). If aninsulation fault occurs in the appliance with this type of protectivecircuit, the current flowing through the outer metal parts which may betouched is led off through the protecting conductor to the neutralconductor. As soon as this shunted current exceeds a preselected currentintensity, the feeding voltage of the appliance is interrupted.

The known protecting circuits do not afford one hundred per cent safetyfor the user if the connection of the protecting conductor is deficient.The occurrence of line and contact resistances at the terminal or plugconnections is unavoidable both with the protecting conductor connectionas well as the neutralization described above. Thus series resistancewhich is not recognized at once may occur between the reference groundand the touchable metal member of the appliance if the contacts arepoor. This series resistance between the touchable metal member and thereference ground may have the consequence of a dangerous voltage beingdeveloped at the metal part if there is a fault in the insulation of theappliance.

For the sake of protecting operators, therefore, there is considerableinterest in continuously watching the grounding resistor (resistancebetween touchable metal part of the appliance and local referenceground). If this ground resistance exceeds a certain critical value, thevoltage supply to the appliance must be interrupted for reasons ofsafety.

For monitoring the grounding resistance mentioned, it is known toconnect the casing of the appliance (or other touchable metal members)to ground potential by a separate additional conductor apart from theconventional connections (main conductor, neutral conductor, and perhapsprotecting conductor).

High-ohmic voltage metering is effected between the protecting conductorand the additional lead. The voltage rises in case of an increase of thegrounding resistance and an insulation fault, and the respectiveappliance is switched off. However, this separate lead to the localreference ground is not available in customary installations so that itcannot be used for every appliance.

It is the object of the invention to provide a circuit for watching thegrounding resistance of electrical appliances which circuit does notrequire an additional lead and with which the ground resistance ischecked continuously at all times.

This object is met, in accordance with the invention, in that one of thelive leads of the appliance is connected to the protecting conductor bya series connection comprising at least one precision resistor formeasuring purposes, a test pulse generator, and a capacitor.

In a preferred modification of the invention the test pulse generatordelivers pulses of constant voltage amplitude. The duration of thepulses generated by the test pulse generator, the precision resistor,and the capacitance of the capacitor are so dimensioned that the timeconstant of the RC member composed of the precision resistor and thecapacitor is much greater than the rise time of the pluses. Thus thecapacitor presents a very low ohmic connection for the pulses ascompared to the other circuit elements. Thus if the grounding resistance(in other words the ohmic resistance between the metallic casing and thereference ground) equals zero, the full voltage amplitude of the pulsesgenerated by the test pulse generator drops at the precision resistor.

On the other hand, if the ground resistance differs from "zero" in caseof a disturbance, a certain voltage drops both across the precisionresistance as well as the grounding resistor. The overall voltage of thetest pulse thus is divided into two partial quantities between which theratio is the same as between the quantities of the resistances(precision resistor and grounding resistance) and the sum of whichcorresponds to the amplitude of the test pulse.

Thus if the voltage drop across the precision resistor is measured by avoltmeter and compared with a preselectable comparative voltage, aconclusion may be drawn as to the grounding resistance from a reductionof the voltage drop across the precision resistor. If the voltage dropat the precision resistor decreases below a given value, a fact whichindicates a rise of the ground resistance above a certain limit value,the voltage supply of the appliance may be interrupted automaticallyand/or a warning means may be released.

The capacitor is designed in accordance with safety regulations so as toafford protection against electric shock upon touching in order towarrant that the protective circuit itself does not feed the touchablecasing with a dangerous voltage even if the protective grounding shouldbe interrupted, in other words at great or even infinitely great groundresistance. In this manner the shunted current caused by the mainsvoltage itself (50 or 60 Hz) with the superimposed current of the testpulse generator remains smaller than the limit value which ispermissible for the human body. To this end, also the voltage amplitudegenerated by the test pulse generator is kept below the thresholdvoltage which is dangerous for the human body.

In a preferred and particularly simple modification of the invention thetest pulse generator is embodied by a trigger diode, such as a diac orfour-layer diode.

Moreover, the test pulse generator may be a transistor circuit which isso designed that great resistance is given if an external voltage belowa certain threshold value is applied, whereas the resistance of thetwo-pole transistor circuit breaks down if the voltage is above thethreshold value.

The invention will be described further below with reference to thedrawing, in which:

FIG. 1 shows a circuit according to the invention;

FIG. 2 is a time diagram of the voltage course at point 1 in FIG. 1 andof the current course through the precision resistor;

FIG. 3 shows possible insulation faults in an electrical appliance, and

FIG. 4 shows a preferred modification of the invention.

In FIG. 1 the electrical appliance equipped with the protective circuitaccording to the invention is marked by reference numeral 12. The casing10 of the electrical appliance 12 is to represent all electricallyconductive structural members which a user may touch. The electricalappliance 12 is connected to the supply mains by a main conductor R anda neutral conductor N. Furthermore, a protecting conductor S is providedwhich connects the casing 10 to the local reference ground M. Thegrounding resistance, in other words the resistance between the casing10 and the local reference ground M is designated R_(E). If the circuitconnection meets the regulations, the grounding resistance R_(E)practically equals zero.

A precision resistor R1 for measuring purposes, a test pulse generatorTG, and a capacitor C_(B) are connected in series between the mainconductor (phase) R and the casing 10.

The test pulse generator TG delivers pulses of constant voltageamplitude. The pulse duration is so dimensioned that the time constantof the precision resistor R1 and the capacitor C_(B) is distinctlygreater than the pulse rise time. The capacitor C_(B) thus presents avery low ohmic connection for the test pulses as compared to the othercircuit components. If it is assumed that the grounding resistance R_(E)has a value of zero ohm, then the full amplitude U_(T) of the pulses ofthe test pulse generator TG drops at precision resistor R1. On the otherhand, if the grounding resistance R_(E) does not equal zero but insteadadopts an infinite value, then the voltage amplitude U_(T) of the testpulse generator TG drops at the precision resistor R1 as well as at thegrounding resistance R_(E) at the ratio of the transistors. In thatevent the following relationships apply:

    U.sub.E +U.sub.1 =constant

    U.sub.E +U.sub.1 =R.sub.T

wherein U_(E) and U₁ are the voltage drops across the groundingresistance R_(E) and the precision resistor R1, respectively.

Thus if the grounding resistance R_(E) rises, while the amplitude of thepulses of the test pulse generator TG remains the same, the voltage dropU₁ across the precision resistor will diminish. Consequently anyundesired rising of the grounding resistance R_(E) above a certain limitvalue can be determined directly from the reduction in the voltage dropU₁ across the precision resistor R1. The reduction in voltage drop U₁ ismeasured by a voltmeter G and compared with a preselected value thereat.As soon as the voltage drop U₁ falls below a predetermined value, thevoltage supply of the electrical appliance 12 or of a certain partthereof is interrupted and/or a warning means is released.

The capacitor C_(B) is designed in accordance with safety regulations soas to afford protection against electric shock upon touching. This isintended to guarantee that no dangerous voltage will be applied by thecircuit to the casing 10 in case of an interruption of the protectivegrounding, in other words at a great or even infinitely great value ofthe grounding resistance R_(E). Consequently, upon touching, the currentled off which is generated by the mains voltage R, N (50 or 60 Hz) aswell as the superimposed mean value of the pulse current generated bythe test pulse generator TG remain smaller than the limit valuecorresponding to the safety regulations. Also the test pulse generatorTG is so designed that the amplitude of the voltage it delivers willremain below an undangerous admissible low tension.

To this end the test pulse generator TG is embodied by a trigger diode,for example a diac. The diac remains high ohmic until the voltageapplied to both its terminals reaches a certain limit value. When thelimit voltage has been exceeded, the diac switches over into a low ohmicstate at which the voltage at its two terminals breaks down to very lowvalues. As soon as the current flowing through the diac fails to reach acertain limit value (holding current), the diac returns to its highohmic initial state.

In FIG. 2 the respective voltage relationships at measuring point 1 ofFIG. 1 as well as the course of the current through the precisionresistor R1 are shown diagrammatically as a function of time. If thevoltage at the main conductor R rises (in sine shape), starting from avalue of zero, at first there will be no voltage drop across precisionresistor R1 nor a voltage change at capacitor C_(B) because no currentwill flow and all the voltage at the diac drops because of the highresistance of the diac (test pulse generator TG). The voltage atmeasuring point 1 and at the diac rises accordingly in parallel with thevoltage of the main conductor R, as shown in the upper curve of FIG. 2.As the voltage at the diac (TG) reaches the preselected threshold valueU_(Sch) the diac switches into its low ohmic state and the voltageacross the diac breaks down, apart from a minor residual voltage U_(RE).At the instant after firing of the diac, the precision resistor R1 ispassed by a current of the magnitude (U_(Sch) -U_(RE)):R1 which isillustrated in the lower curve in FIG. 2. Recharging of the capacitorC_(B) is effected by this current having the time constant R1×C_(B), thecurrent intensity dropping exponentially, as shown at the bottom in FIG.2, until the holding current of the diac is failed to be reached,whereupon the diac returns into its high ohmic state. Following that,the cycle begins again and the voltage at measuring point 1 rises withthe voltage at the main conductor R until the crest value of thehalfwave of the sinusoidal mains voltage is reached. The capacitor C_(B)then will have been charged to a voltage which lies between the fullpeak voltage of the mains halfwave and the value which is diminished bythe threshold voltage U_(Sch). Upon surpassing of the crest value, thesame process is repeated at inverse polarity until the crest value ofthe negative halfwave has been reached, etc. The integral ∫i (t)×dt ofthe pulses corresponds to the value which would have been reached if thecapacitor C had been connected to the ground terminal M without theinterposition of the diac (TG). In this way it is safeguarded that ahazard of persons suffering an electric shock cannot be given when useis made of a capacitor which is protective as described.

If a ground resistance R_(E) of finite magnitude exists between the diacand the local reference ground M, the voltage amplitude of the testpulses U_(T) =U_(Sch) -U_(RE) splits up between precision resistor R1and grounding resistance R_(E), as explained above, and the timeconstant at which the current amplitude of the pulses drops, grows inaccordance with the value C_(B) ×(R1+R_(E)), whereas the integral ∫i(t)×dt remains unchanged. The meter G monitoring the voltage dropsacross the precision resistor R1 and releasing an alarm when a limitvalue is failed to be reached, may be fed with voltage also directlyfrom the mains connection without connecting a transformer in between.

With the circuit shown in FIG. 1 it is assumed that the only conductiveconnection between the protecting conductor S and the main conductor Ror the neutral conductor N is established by way of the "true earth",i.e. the grounding of the protective conductor S at the place of theappliance and the grounding of the neutral conductor at a differentplace within the supply network of the power supply company.

In accordance with FIG. 3 it is conceivable that an insulation faultresistor R_(F) for instance connects the main conductor R with thecasing 10 of the appliance 12 or that an insulation fault resistorR_(F') connects the neutral conductor N in that manner. Such a finiteinsulation fault resistor R_(F) or R_(F') would be connected parallel tothe circuit R1, TG, and C_(B) between the main conductor R or theneutral conductor N, respectively, and the protecting conductor S. It isconceivable that the grounding resistance R_(E) becomes infinitely greatand the insulation fault resistor R_(F) or R_(F') approaches zero as ashortcircuit. In this event the protective circuit would provideerroneous results in that it would indicate perfect protectivegrounding.

To avoid such an erroneous indication, a protecting conductor choke L isconnected between the input of the protecting conductor S to which thecapacitor C_(B) is connected directly and the casing 10 of theappliance, as shown in FIG. 4. The impedance of this choke is small forthe mains frequency (50-60 Hz) so that it does not present anyconsiderable voltage drop for a shunted current. However, for the shortpulses of the test pulse generator TG the impedance of this protectingconductor choke L which is connected in series with the insulation faultresistor R_(F) or R_(F') is so high that the ground resistance R_(E)practically can be measured truthfully.

What is claimed is:
 1. A circuit for the continuous watching of thegrounding resistance of an electrical appliance having a casing, aprotecting conductor and a live lead, the electrically conductive partsof which that may be touched, including the casing of the appliancebeing connected to a ground potential by said protecting conductor,comprising a series connection connecting said live lead to saidprotecting conductor and including at least one precision resistor formeasuring purposes, one test pulse generator providing pulses with apulse rise time, and one capacitor connected to said precision resistorproviding a time constant greater than said pulse rise time.
 2. Thecircuit as claimed in claim 1 wherein the test pulse generator deliversvoltage pulses of constant amplitude.
 3. The circuit as claimed in claim2 wherein the duration of the pulses generated by the test pulsegenerator, the precision resistor, and the capacitance of the capacitorare so dimensioned that the time constant of the RC member composed ofthe precision resistor and the capacitor is great as compared to thepulse rise time.
 4. The circuit as claimed in claim 3 wherein thevoltage drop across the precision resistor is measured by a voltmeterand compared with a preselectable comparative voltage.
 5. The circuit asclaimed in claim 4 wherein the feeding of voltage to the electricalappliance or part thereof is interrupted when the voltage drop measuredby the voltmeter drops below the comparative voltage.
 6. The circuit asclaimed in claim 5 wherein a warning means is released when the voltagedrop measured by the voltmeter drops below the comparative voltage. 7.The circuit as claimed in claim 6 wherein the capacitor is designed toafford protection against electric shock.
 8. The circuit as claimed inclaim 7 wherein the capacitor allows for the leakage current caused upontouching by the mains voltage as well as the pulse current caused by thetest pulse generator remain smaller than the limit value admissible forthe human body even if the ground connection is interrupted between theelectrically conductive parts and the ground potential.
 9. The circuitas claimed in claim 8 wherein the amplitude of the voltage generated bythe test pulse generator lies below the limit value admissible for humancontact.
 10. The circuit as claimed in claim 9 wherein the test pulsegenerator includes a trigger diode.
 11. The circuit as claimed in claim9 wherein the test pulse generator includes a transistor circuit whichis high ohmic upon application of an external voltage below a certainthreshold value and low ohmic upon application of an external voltageabove the certain threshold value.
 12. The circuit as claimed in claim11 and further comprising a protecting conductor choke connected betweenthe electrically conductive parts and the protecting conductor, theimpedance of the choke being small at mains frequency but great for thetest pulses.
 13. A circuit for continuously monitoring the groundresistance of an electrical appliance having a casing, a protectingconductor and a live lead, the casing being connected to a groundpotential by the protecting conductor, the circuit comprising:aprecision resistor; a test pulse generator connected to said precisionresistor and providing pulses with a pulse rise time; a capacitorconnected to said precision resistor providing a time constant greaterthan the pulse rise time; and, the circuit being serially connectedbetween the live lead and the protecting conductor.
 14. The circuit ofclaim 13 wherein the pulses have a constant voltage amplitude.
 15. Thecircuit of claim 14 wherein the parameters of said test pulse generator,said precision resistor, and said capacitor are such that the timeconstant of the RC member composed of said precision resistor and saidcapacitor is great compared to the pulse rise time.
 16. The circuit ofclaim 15 wherein the voltage drop across said precision resistor ismeasured by a voltmeter and is compared with a preselectable comparativevoltage and wherein voltage to the electrical appliance is interruptedwhen the voltage drop drops below the comparative voltage.
 17. Thecircuit of claim 16 wherein a warning means for alerting a person isreleased when the voltage drop drops below the comparative voltage. 18.The circuit of claim 17 wherein said capacitor allows for the leakagecurrent caused upon touching and supplied by the mains voltage and forthe pulse current caused by said test pulse generator to remain smallerthan the limit value admissible for the human body even if the groundconnection is interrupted between the electrically conductive parts andthe ground potential and wherein the amplitude of the voltage generatedby said test pulse generator lies below the limit value admissible forhuman contact.
 19. The circuit of claim 18 wherein said test pulsegenerator includes a trigger diode.
 20. The circuit of claim 19 whereinsaid test pulse generator further includes a transistor circuit which ishigh ohmic upon application of an external voltage below a certainthreshold value and low ohmic upon application of an external voltageabove the certain threshold value.
 21. The circuit of claim 20 furthercomprising a protecting conductor choke connected between theelectrically conductive parts and the protecting conductor, theimpedance of the choke being small at mains frequency but great for thetest pulses.